Bottom-up and top-down manipulations for multi-order photonic crystallinity in a graphene-oxide colloid

The recent discovery of structural coloration in aqueous graphene-oxide (GO) dispersions has increased the applicability of carbon-based two-dimensional materials. However, the origin of the photonic band-gap is still poorly understood, and its practical manipulation is still in an early developmental stage. Here, we demonstrate full-color reflection with first- and second-order Bragg reflections in a GO dispersion, and we use two fundamental approaches to manipulate GO photonic crystals, namely, bottom-up and top-down manipulation by controlling the Debye length and using shear or surface fields, respectively. Nanoscopic tailoring of the electrostatic effective thickness and macroscopic smoothing of the curvature of the GO sheet result in similar modifications of the quality and pitch of the photonic crystallinity. Direct observation of the GO particle alignments reveals excellent electrostatic layer-to-layer packing assembly and rather poor in-layer assembly. These results elucidate the mechanism that governs the nematic nature of GO (rather than its lamellar mesophase) and the origin of its photonic crystalline periodicity and provide new methodologies for exploiting these attractive features in actual applications. Dispersions of graphene oxide nanoflakes in water can be manipulated into photonic crystals that reflect the full spectrum of visible colour. Normally, dissolving nanometre-thin graphene oxide crystals in water produces brownish solutions. Jang-Kun Song from Sungkyunkwan University in South Korea and co-workers used extensive oxidation and cleaning procedures in combination with bottom-up and top-down control strategies to coax unexpected optical activity from the irregularly shaped nanoflakes. The team's synthetic procedure produced a strong electric double layer on graphene oxide surfaces that induced the particles to stack into periodic, light-reflecting structures similar to cholesteric liquid crystals. Adding salt ions to the watery dispersion allowed the photonic structures, and hence the colour reflections, to be adjusted at molecular scales. Alternatively, the researchers could ‘paint’ glass slides by using shear forces produced by stirring to direct colour output. We demonstrate full-color reflection with first- and second-order Bragg reflections in a GO dispersion, and we use two fundamental approaches to manipulate GO photonic crystals, namely, bottom-up and top-down manipulation by controlling the Debye length and using shear or surface fields, respectively. Direct observation of the GO alignments using confocal microscopy reveals excellent layer-to-layer packing assembly and poor in-layer assembly, elucidating the nematic nature of GO rather than its lamellar mesophase. In addition, modulations of the local GO density and alignment enable us to demonstrate full-range color paintings, including chromatic and achromatic colors.